(1) Field of the Invention
This invention relates to an optical transmission device for bi-directional optical communications, which performs bi-directional optical communications by transmitting optical signals having different wavelengths in two ways.
(2) Description of the Related Art
For a transmission type communication system designed to construct a multimedia network, an optical communication system capable of increasing transmission capacity has been used in recent years. Various multiplexing systems have been available in order to attain extra-large capacity in the optical communication system. Such multiplexing systems include, for example, a time-division multiplexing (abbreviated as TDM) transmission system, an optical time-division multiplexing (OTDM) transmission system, a wavelength-division multiplexing (WDM) transmission system, and so on.
Among these multiplexing systems, the WDM transmission system is promising, because it facilitates flexible construction of an optical communication network (lightwave network) by using an erbium-doped optical fiber amplifier (EDFA) having a wide gain band so as to perform optical level cross connection, branching/insertion of optical signals or multiplexing of various services.
In the lightwave network employing such a WDM transmission system, for example, an optical add/drop multiplexer (ADM) is used. This optical ADM includes transit points called nodes provided in the middle way of a transmission line, and enables free branching or insertion of optical signals by selectively transmitting, among multiplexed optical signals, an optical signal having a specified wavelength through the nodes and receiving the other optical signals having wavelengths different from the specified one by the nodes.
In other words, in the WDM transmission system, by utilizing the optical ADM, a speed for performing transmission among the nodes can be increased, and the network can be made flexible.
The lightwave network employing the foregoing WDM transmission system typically uses a uni-directional wavelength-division multiplexing system, which transmits optical signals by using optical fibers [single mode fiber (SMF)] individually allocated to clockwise and counterclockwise directions.
Referring to FIG. 19, there is shown an example of a wavelength-division multiplexing communication system 50 which employs the foregoing uni-directional wavelength-division multiplexing system. The wavelength-division multiplexing system 50 shown includes two opposing optical transmission devices 501 and 502 which are connected to each other by two uni-directional optical fibers 81A and 81B.
Specifically, between the optical transmission devices 501 and 502, optical signal transmission is performed as follows. Optical signals (.lambda..sub.1 -.lambda..sub.8) are transmitted from the optical sender (OS) 80A of the optical transmission device 501. These optical signals are then received through the optical fiber 81A by the optical receiver (OR) 82A of the optical transmission device 502. Optical signals (.lambda..sub.1 -.lambda..sub.8) are transmitted from the OS 80B of the optical transmission device 502. These optical signals are then received through the optical fiber 81B by the OR 82B of the optical transmission device 501.
Thus, in the wavelength-division multiplexing communication system 50 shown in FIG. 19, the optical fibers 81A and 81B are individually allocated to the two directions of the optical signals. All the optical signals transmitted through each of the optical fibers 81A and 81B are transmitted in the same direction.
However, in the wavelength-division multiplexing communication system 50, it may become impossible to perform normal communications between the optical transmission devices 501 and 502. This problem occurs because the transmission of the clockwise optical signals is interrupted if, among the two optical fibers 81A and 81B, for example, as shown in FIG. 20, a failure occurs in one optical fiber 81A and the communications are cut off (may be referred to as communication cut-off, hereinafter).